In particular, the invention concerns a shroudless blade or vane for use in the compressor or fan of an axial flow gas turbine engine.
Each stage in a compressor comprise an array of rotating blades and static vanes that are orientated relative to each other to raise the static air pressure while maintaining axial velocity at a near constant level. The compressor is designed so that when operating at its working line the rate of deceleration, or diffusion, of the airflow through each of the blade or vane stages is limited to avoid flow separation and subsequent surge, stall or flutter.
In a rotor of shroudless blades there is simply a clearance gap between the tip end surface of each blade and the inner surface of the rotor casing wall. Each blade aerofoil turns incident airflow, in its own frame of reference, and in doing so generates aerodynamic lift, which produces a pressure difference between the opposing side surfaces of the aerofoil. Hence, these side surfaces are often referred to as pressure and suction surfaces. The tip clearance gaps provide a gas leakage path between the higher pressure side of the aerofoil (pressure surface) and the lower pressure side (suction surface). This effect is analogous to an electrical “short circuit”.
FIG. 1 of the accompanying drawings provides a schematic illustration of this leakage flow and its development in a compressor rotor. A portion of a compressor rotor is shown comprising two shroudless compressor blades, indicated generally at 2, and a portion of the rotor hub 4. In the drawing the direction of the main gas flow is generally away from the viewer, so that the suction surfaces 6 of the compressor blades are facing the viewer and the pressure surfaces 8 are on the hidden sides of the blades. The wall of the compressor casing is omitted for clarity so that the tip clearance distance is not directly depicted. However, as previously mentioned such a clearance exists between the tips 10 of the compressor blades 2 and the internal surface of the casing and produces overtip clearance flows indicated at 12 in the illustration. In the immediate vicinity of the aerofoil suction surfaces 6 the overtip leakage flows 12 interact with the main gas flow and generate clearance vortexes 14. This mixing of the leakage flow and main gas reduces the aerodynamic efficiency of the rotor stage and can be an important factor in the causes of stall and flutter.
According to GB Patent No. 736,835 published in 1955, it was already known in the field of propeller design for use on liquid media to provide apertures near the tips of propeller blades in order to promote a bleed flow from the high pressure side of the blade to the low pressure side in an attempt to fill up the spaces brought about by cavitation. Moreover, it was acknowledged that it was also known in the field of flow machines for gaseous mediums to generate a secondary flow from the high pressure to the low pressure side of blades by means of slots extending the whole height of the blade. However, such slots severely comprised the mechanical strength of the blade and were disregarded for machines operating at high speeds of rotation. In order to combat the secondary flow effects while retaining the mechanical strength of the blades GB 736,835 proposed to form in the body of a shroudless blade apertures in zones near the casing wall for the purpose of bleeding a proportion of the flow from the high-energy (high pressure) side of the blade to the low-energy (low pressure) side. The apertures could be in the form of simple slots or diagonally extending bores, having a depth of approximately ⅕th of the overall height of the blade body. Slots of such depth have not been adopted, at least in part due to the higher mechanical strength required of more recent, higher performance blade designs.